Method of stimulating well production by explosive-induced hydraulic fracturing of productive formation



March 23, 1965 H. H. MOHAUPT 3,174,545

' METHOD OF STIMULATING WELL PRODUCTION BY EXPLOSIVE-INDUCED HYDRAULICFRACTURING OF PRODUCTIVE FORMATION Filed Jan. 13. 1958 2 Sheets-Sheet 1T w p M RM .9 OH m W 2 M 3 HT M am V. M F

R HH: .2 a:

H. H. MOHAUPT 3,174,545

PRODUCTION BY EXPLOSIVE-INDUCED OF PRODUCTIVE FORMATION March 23, 1965METHOD OF STIMULATING WELL HYDRAULIC FRACTURING Filed Jan. 1:5. 1958 2Sheets-Sheet 2 /N VENTOR.

HENRY H. MOHHUPT BY H/S ATTO'KNEYS. HARP/3, K/ECH, FOSTER 8; Heme/sUnited States Patent Office meme, jififi tion of Texas Filed Jan. 13,1958, Ser. No. 708,481 8 Claims, c1. 166-36) The present inventionrelates in general to a method of stimulating wells, such as oil wells,to increase production therefrom and, more particularly, to a method ofincreasing the permeability of a productive formation into which a Wellbore extends so as to increase the flow of oil, or other fluids, fromthe formation into the well bore.

A primary object of the invention is to provide a method which fissuresthe productive formation generally radially outwardly from the well borethrough substantial distances in a controlled manner with little or noshattering or compaction of the formation. Thus, the invention increasesthe permeability of the formation to increase the productivity thereofwithout adversely affecting the overall consolidation of the formation,which is an important feature.

More specifically, a primary object of the invention is to producecontrolled fissuring of the productive formation by burning a submergedcharge of a nondetonating explosive of the propellant type in anirregular, but controlled, manner so that, as the explosive burns, itproduces in the liquid adjacent the productive formation andcommunicates to the formation pressure pulsations of sufficientmagnitude to fissure the formation in a controlled manner, but ofinsufficient magnitude to shatter or otherwise destructively affect theformation.

The rapid pressure oscillations caused by the charge are transmitteddirectly, or through a casing, into the formation where they causeminute and repeated shiftings of the formation layers with respect toeach other. The formation is simultaneously subjected to the pulsatingfluid which tends to force fluid into the minutely shifting interfacesof the formation. In a cased well, this occurs at the point or pointswhere the casing has been provided with perforations of adequatediameter. This action es tablishes permanent breaks in the formationbond and one or more actual fissures result which are extended away fromthe well bore as the burning of the explosive continues, and more andmore well fluid is thus forced into them. These fissures can be furtherextended by the application of additional charges in the vicinity of thezone. They can also be extended by acidizing or by similar methods, ifdesired.

Another object of the invention is to produce nonuniform combustion ofthe propellant explosive by igniting the charge in such a manner thatthe explosive is dispersed in the liquid in the well bore adjacent theproductive formation in discrete form and is caused to move back andforth axially of the well bore as it burns to produce constantlyshifting zones of maximum combustion intensity, thereby producingshort-duration pressure pulsations in the liquid which cause minutefissures in the formation.

Another object is to control the rate of combustion of the propellantexplosive by regulating the size of the explosive particles or grains,the ignition intensity and pattern, the mass of explosive, the nature ofand the head of liquid in the well, and by using an inert or semiactivefiller or matrix in which the explosive particles are embedded. Byvarying some or all of these factors, the frequency and amplitude of thepressure pulsations produced in the liquid to fissure the formation maybe varied.

A further object is to provide a propellant explosive which is eithergranular, or which is capable of being broken up into smaller, discreteparticles, so that, upon ignition of the explosive charge, mixing of theexplosive with the liquid in the well bore is obtained. With such amixture, combustion of the explosive particles takes place in localizedzones, thereby creating pressure ceriters which cause the mixture tomove back and forth axially of the Well bore to create new high pressureCell-f ters and new zones of localized combustion, the over-all resultbeing the creation of pressure pulsations in the liquid which fissurethe formation in a controlled manher. In the foregoing combustionprocess, many of the explosive particles are ignited, extinguished,reignited, and so on, until substantially all of the particles have beenburned, thereby prolonging the period during which pressure pulsationsare developed in the fluid and applied to the formation to fissure it.Also, many of the explosive particles may be driven into the fissure orfissures and burned therein to enhance the fissuring action.

Another object of the invention is to obtain nonuniform ignition of thetotal charge by igniting it at one end only, by igniting it at itscenter throughout a substantial portion of its length, by igniting it ata plurality of axially spaced points, by igniting it longitudinally butnon-axially, and the like.

An object of the invention in connection with a charge of propellantexplosive which includes a matrix having the explosive particles orgranules embedded therein is to control the length of the combustionperiod, and the amplitude of the pressure pulsations in the liquid, byvarying the proportions of explosive and filler, combustion progressingrelatively rapidly with a low proportion of filler and relatively slowlywith a high proportion of filler.

An important object is to submerge the charge of propellant explosivebeneath a substantial head of liquid to obtain pressure pulsations of amagnitude sufficient to produce the desired fissuring of the productiveformation. In order to obtain satisfactory fissuring of the productiveformation, it is essential that the formation and the charge ofpropellant explosive be submerged beneath a head of liquid at leastequivalent to 200 feet of liquid having a specific gravity in theneighborhood of one. In other words, theexplosive charge should bepositioned opposite the productive formation to be fissured beneath ahead of at least 200 feet of oil or water, or a mixture thereof, toobtain proper fissuring, a lesser head being possible where the densityof the liquid is above unity due to the presence of drilling mud, forexample. The head of liquid may range upwardly from 200 feet of oil orwater to several thousand feet of oil or water, or more, a head of inthe neighborhood of 1,000 feet of oil or water being preferred.

With heads from about 200 feet to about 600 feet of oil or water, theliquid must be unconfined by packers, or the like, for several hundredfeet thereabove so as to provide above the liquid a large reservoir ofair or gas capable of absorbing excessive pressure pulsations. However,with a liquid head of more than about 600 feet, the compressibility ofthe liquid and the elastic deformability of the casing and the formationmake nonconfinement of the liquid unnecessary.

An additional object of the invention is to utilize a propellant chargein the form of a long, slender cylinder having a diameter which is lessthan the diameter of the well bore to provide a liquid space alongsidethe charge for the purposes of the combustion process hereinbeforedescribed, this liquid space being an annulus if the charge is centeredin the well bore. For a well bore of about 4 inches in diameter, ormore, the diameter of the propellant charge should be approximately 3inches. The length of the charge of propellant explosive is madesufficient to accommodate the total mass necessary to produce thedesired fissuring of the formation and it is essential that the lengthof the charge be suflicient to produce the desired axial accelerationand migration of the explosive particles incident to the combustionprocess hereinbefore discussed.

An important object of the invention is to utilize a propellant chargehaving a lengthdiameter ratio of at least 3:1 to obtain the desiredparticle migration axially of the well bore, a length-diameter ratioless than about 321; being insufiicient to produce the necessary axialparticle movement. The length-diameter ratio may run as high as 40:1 fora charge which is ignited at one end only, and may substantially exceed40:1 if the charge is ignited substantially throughout its entirelength, or at a plurality of points throughout its length. With ignitionat one end only, any length-diameter ratio in excess of 40:1 tends toproduce excessive migration of the propellant particles with consequentexcessive pressure pulsations in the liquid. However, by igniting thecharge in other places, as well as at one end thereof, zones oflocalized combustion are produced which limit axial movement of theexplosive particles dispersed into the liquid, and which thus limit thepressure peaks produced in the liquid.

An important object of the invention in instances wherein it ispracticed in a cased well bore is to concentrate the effects of thepressure pulsations in the liquid by locating the charge opposite one ora few large holes in the casing which lead to the formation to befissured, such hole or holes being much larger than any conventionalperforations with which the casing may be equipped and being at least 1inch, and preferably from 1 /2 to 2 inches, in diameter. However, theinvention may be practiced with only the conventional casingperforations provided the total area of such perforations isconsiderably greater than that of a single hole of the size rangeindicated.

An important object of the invention is to utilize an apparatus whichincludes means for lowering the charge of propellant explosive intoposition opposite the productive formation to be fissured and forigniting the charge in such position, and which includes meansresponsive to actuation of the igniting means for disconnecting thenon-expendable portions of the apparatus from the charge itself so thatsuch nonexpendable portions may be raised in the well bore to a positionof safety, and to a position wherein they do not interfere with thecombustion of the charge, before actual combustion of the charge isinitiated.

The foregoing objectives, advantages, features and results of thepresent invention, together with various other objects, advantages,features and results thereof which will be evident to those skilled inthe art in the light of this disclosure, may be attained with theexemplary embodiments of the invention illustrated in the accompanyingdrawings and described in detail hereinafter. Referring to the drawings:

FIG. 1 is a vertical sectional view illustrating in a well bore anapparatus of the invention for carrying out the well stimulation methodor process thereof;

FIG. 2 is an enlarged, vertical sectional view showing a charge ofpropellant explosive of the invention, and an igniting means therefor,positioned in the well bore;

FIG. 3 is a view similar to FIG. 2, but showing the charge of propellantexplosive after ignition;

FIG. 4 is a view similar to FIGS. 2 and 3, but showing the charge aftercompletion of combustion;

FIG. 5 is a view similar to FIG. 2, but illustrating another embodimentof the invention;

FIG. 6 is a transverse sectional view taken along the arrowed line 6-6of FIG. 5;

FIG. 7 is a vertical sectional view of another embodiment of a charge ofpropellant explosive of the invention;

FIGS. 8 and 9 are fragmentary sectional views illustrating otherpropellant-explosive charges of the invention; and

PEG. 10 is a view similar to FIG. 7, but illustrating anotherpropellant-explosive charge of the invention.

Referring particularly to FIG. 1 of the drawings, illustrated therein isa well bore 10 which extends from the surface 12 through overlyingformations 14 into or through a productive formation 16 the permeabilityof which is to be increased by controlled fissuring generally radiallyoutwardly from the well bore in accordance with the invention. In theparticular installation illustrated, the well bore It) is cased in theusual manner by a casing 13 which may be provided with perforations 2tcommunicating with the productive formation 16. If desired, the casing18 may be provided above the surface 12 with a blowout preventer 22 bymeans of which flow from the well may be shut off after completion ofthe method of the invention if necessary.

It is essential to the invention that there be in the well bore 19 abovethe productive formation 16 to be fissured, a body of liquid 24providing a head at least the equivalent of a head of 200 feet of liquidhaving a specific gravity of about one. If there is present in the wellbore 1t oil or water, or a mixture thereof, extending about 200 feetabove the productive zone 16, this will provide the necessary minimumhead. Otherwise, oil, water, or other liquid must be introduced into thewell bore to provide a head equal to about 200 feet of oil or water. Thehead of liquid above the productive formation 16 is preferablyconsiderably in excess of 200 feet and may range upwardly to severalthousand feet, or more, a preferred value being at least 1,000 feet. Theliquid 24 must be unconfined to provide a surge chamber thereabove tolimit the pressures developed in the liquid if the head is less thanabout 600 feet. The well bore 10 may be left open for this purpose but,if any blowout preventer, such as 22, or any lubricator, is used, itmust be at least several hundred feet above the surface of the liquidunder such conditions. With heads exceeding 600 feet, nonconfinement isnot absolutely essential due to the compressibility of the fluid and theelastic deformability of the casing and formation.

In order to concentrate the fissuring action of the invention in a casedwell bore, the casing 18 is provi with at least one large hole 26opposite the productive formation 16 with any suitable apparatus,although more than one may be provided. This hole is large as comparedto any perforations 253 with which the casing 18 may be provided, andmust be at least 1 inch, and prefer ably from 1 /2 to 2 inches, ineffective diameter to minimize throttling of surges of the liquid 24therethrough. The term effective diameter is utilized to describe theequivalent diameter of a hole 26 which may not be perfect- 1y circular.When the invention is practiced in an uncased well bore, as illustratedin FIGS. 5 and 6 of the drawings, the concentrating effect of the singlelarge hole 25 in the casing 18 is not attained, but satisfactoryfissuring of the productive formation is nevertheless achieved.

The invention further involves positioning in the well bore 10 oppositethe productive formation 16, and opposite the hole 26 if the casing 18is present, a long, slender charge 28 of a propellant explosive, thedetails of the charge 28 being set forth hereinafter. It is importantthat the charge 28, which is preferably cylindrical, have a diameterconsiderably less than the diameter of the well bore in, or considerablyless than the inside diameter of the casing 18, to insure the presenceof liquid alongside the charge for the purpose of producing pressurepulsations in the liquid 24- in a manner to be described. The liquidalongside the charge 28 will have more or less the form of an annulus,depending upon whether the charge is centered in the well bore, or islocated to ne amassside of center. Normally, the charge 28 is supportedat least approximately in the center of the well bore in a manner to bedescribed, but irregularities in the well bore may not permit attainingexactly a centered position. In order to insure the presence of anadequate quantity of liquid alongside the charge 2%, the diameter of thecharge is not more than about 3 inches for a well-bore diameter, orinside casing diameter, of about 4 inches.

In order to provide the total mass of propellant explosive necessary toachieve the desired fissuring of the formation 16, and in order toprovide the charge 23 with sufiicient axial length to permit thenecessary axial acceleration and movement of the propellant particleswhich Will be discussed hereinafter, the charge 28 must have alength-diameter ratio, i.e., a ratio of length to diameter, of not lessthan 3: 1. Preferably, the length-diameter ratio of the charge 28 isconsiderably in excess of 3:1, although it should not exceed about 40:1where the charge is ignited at one end only, as hereinafter describedHowever, where the charge 28 is ignited at points other than one endthereof, as will also be described hereinafter, the length-diameterratio may exceed 40:1. Limiting the length-diameter ratio to not morethan about 4021 with one-end ignition of the charge 28 is necessary toprevent excessive migration of propellant particles axially of the wellbore 1%, and thus to prevent excessive pressures. However, with ignitionat a point or points other than at one end of the charge 28, axialpropellant migration, and thus the resulting pressure pulsations, arereduced so that, under such conditions, length-diameter ratios in excessof 40:1 may be utilized.

As previously suggested, the charge 28 comprises an explosive of thepropellant type, i.e., an explosive which is slow burning andnondetonating, explosives of this type being well known in the art sothat it is unnecessary to discuss specific examples herein. The charge28 should consist of a granular propellant mass made up or" small grainsor particles, or, if a solid mass, it should be readily frangible byinitial combustion of the charge so that the charge is immediatelyreduced to small particles, such small grains or particles beingnecessary to permit thorough dispersion in the liquid 24 and to permitthe gases formed as a result of the combustion of the explosive to riseupwardly through the liquid 24 in the well bore 10 without substantialentrainment of the explosive particles in the rising gases.

In some instances, the charge 28 may consist of an unenclosed mass ofpropellant particles or grains embedded in a matrix, filler or binderwhich is self-supporting, but which is suificiently frangible thatinitial combustion of the charge breaks it up into small piecesincapable of substantial entrainment in the rising gases. In mostinstances, however, the charge 28 is enclosed by a readily frangiblecontainer 30, the propellant explosive being utilized in such cases withor without a filler or matrix.

The container 30, which is shown in FIGS. 2 to 4 of the drawings,includes a cylindrical tube or shell 32 the upper and lower ends ofwhich are closed by an upper, firing head 34 and a lower head 36,respectively. A similar container 38 is shown in FIG. 5, this containeralso including a shell 4t closed by an upper, firing head 42 and a lowerhead 44 and containing a charge 46 of a propellant explosive. FIGS. 7and 10 illustrate other similar containers 48 and 49 comprisingcylindrical shells t and 51 closed by upper heads, not shown, and lowerheads 52 and 53 and containing charges 54 and 55 of propellantexplosive. The charges 46, 54- and 55 are similar to the charge 28 andeverything discussed herein in connection with the charge 28 is equallyapplicable to the charges 46, 54 and 55.

The shells 32, 40, 5t and 531. are formed of a frangible material whichis readily broken up into small pieces, such as the pieces 56 in FIG. 3,upon initiation of combustion of the charges 28, 46, 54 and 55 in amanner to be described. The charge-containing shells may be formed of 6various materials, such as aluminum or plastic, which will break up intorelatively small pieces with the thin shells shown, such materials beingreadily drillable during any subsequent drilling operations which may beperformed. Alternatively, the charge-containing shells may be formed ofsuch highly frangible materials as glass, ceramics, and the like. Suchmaterials will shatter into very small fragments upon initiation ofcombustion of the charges con tained therein, and are readily drillablealso. As another alternative, the shells may be formed of a frangiblemate rial, such as nitrocellulose, which will burn when ignited bycombustion of the charge contained therein. Still another alternativematerial for the charge-containing shells is a combustible plastic whichmay be burned by incorporating in the charge contained in such shell amaterial which liberates oxygen when heated by combustion of thepropellant explosive. For example, such oxygenliberating materials asnitrates, chlorates, or perchlorates, may be used.

In order to minimize the thickness of the shell 32, for example, tominimize the amount of debris left in the well bore 10, or to facilitatecombustion of the shell if it is made of a combustible material, thelower head 36 is a piston which is slidable axially of the shell 32 andwhich is sealed relative thereto by an O-ring 58, the piston 36 beingretained within the shell in any suitable manner, as by rolling orpeening the lower end of the shell inwardly, as indicated at an. As willbe apparent, with this construction, as the charge 28 is lowered intothe liquid 24 in the well bore 116, the piston 36 is moved upwardly bythe external pressure applied to the container 30 to compress thepropellant and thus tend to equalize the internal and external pressuresapplied to the container. Such pressure equalization, or tendency towardpressure equalization, minimizes the tendency of the shell 32 tocollapse and thus permits the use of a thinner, more readily disposableshell. A similar construction is shown in FIG. 7, wherein the lower head52 is a piston sealed with respect to the shell 59 and by an O-ring 62.In FIG. 5, the lower head 44 is immovable, being sealed by an O-ring 64.

To facilitate internal and external pressure equalization, ashereinbefore discussed in connection with FIGS. 2 to 4 and FIG. 7 of thedrawings, the voids in the mass of propellant grains or particlesforming the charges 28 and 54 may be filled, or partially filled, with afluid, or semifluid, filler, such as parafiin, grease, inert powder, orthe like. Such fluid, or semifluid, filler facilitates equalizing theinternal and external pressures.

In a charge like the charge 46 shown in FIG. 5, which is enclosed in ashell it) provided with fixed upper and lower heads 42 and 44-, thethickness of the shell 49 may be minimized by embedding the propellantgrains or particles in a rigid matrix which is shatterable by initiationof combustion of the charge. For example, such a matrix may be plasterof Paris, sulphur, plastic, or the like. Alternatively, the propellantparticles may be held together by a suitable cementing agent. In someinstances wherein the propellant particle are embedded in a matrix ofthe foregoing nature, the matrix itself may be utilized to support thepropellant particles Without an enclosing container. For example, undersuch conditions, at least the shell 4% and the lower head 44 of FIG. 5may be omitted.

As hereinbefore suggested, an important feature of the invention is thateach propellant charge is caused to burn at an irregular rate, duepartly to the manner in which the charge is ignited, partly todispersion of the charge in the liquid 24 adjacent the formation 16 tobe fissured, and partly to other factors which will be discussed here;-inafter. Considering now the manner of ignition, the charges 28, 46 and54 are shown as ignited in diiferent ways, the means for igniting thecharge 23 being considered first for convenience.

The upper head 34 of the container 30, which will be referred to as afiring head hereinafter, is provided with an axial bore 6d whichcontinues as an axial bore 68 in a bushing '70 threaded into the firinghead. The upper end of the bore 66 contains an ignitor '72 which isfired in a manner to be described, this ignitor setting off a delaytrain 74 in the bores as and 68. Gases formed by combustion of the delaytrain "74 are expanded, cooled and vented into the charge 23 through apassage '76 in the firing head 34-. Ultimately, the delay train 74reaches and fires a detonator 7?; in the bore 68, this detonator, inturn, igniting a high explosive train 819 which extends along the axisof the charge 28 substantially throughout the entire length thereof.With tiis construction, progressive ignition of the center of the charge28 occurs along the axis of the charge, such progressive centralignition of the charge 23 resulting in irregular ignition of the totalcharge and in outward expansion which fragmentizes the shell 32 andwhich disperses the propellant throughout the liquid 24 surrounding thecharge. As a result of this dispersion, the propellant explosive isburned at an irregular rate for reasons which will be discussed in moredetail hereinafter.

Referring to FIG. of the drawings, the upper head 42 of the container38, which will also be referred to hereinafter as a firing head, isprovided with a bore 3?. which continues as a bore 84 in a bushing 86,the bore 82 containing an ignitor 83 and the bores 82 and 84 containinga delay train 99 gases resulting from the combustion of which are ventedinto the charge 46 through a passage 2 in the firing head. In thisconstruction, the delay train 90 communicates directly with the upperend of the charge 4-6, only the upper end of the charge being ignited inthis embodiment. Ignition of the upper end of the charge results inexpansion to fragmentize the shell 40 and disperse the propellantparticles forming the charge 45 throughout the liquid 24 in the wellbore it). Again, such dispersion results in irregular cornbustion, ashereinafter described.

In EEG. 7 is shown an igniting means which includes an explosive train)4 similar to the explosive train 80, except that it is provided ataxially spaced points with nodes 915. The nodes 96 may be faster orslower burning than the explosive train 94 itself, but, in any event,they provide points of more intense ignition of the charge 54 and thusaccelerate the dispersion of the charge in the liquid 24 at axiallyspaced points.

PEG. shows an igniting means comprising a high explosive train 97 whichextends longitudinally, but nonaxially, through at least part of thelength of the charge 55', the train 7 being offset relative to the axisof this charge to produce irregular ignition.

Considering the irregular nature of the combustion of the charges 28,4d, 54 and 35, these charges are all ignited nonuniformly, the ignitionof the charge 28 being nonuniform due to the fact that the explosivetrain 80 ignites only the propellant particles at the center of thecharge, the charge 46 being ignited nonuniformly since it is ignited atits upper end only, the charge 5'4 being ignited nonuniforrnly along itscenterline by the explosive train 94- and the nodes 96 thereon, and thecharge 55 being ignited nonuniformly since it is ignited along one sidethereof. Such nonuniform ignition of the charges results infragmentization of the shells containing them and dispersion of theparticles forming the charges throughout the liquid 24 adjacent theproductive formation 16. The dispersed charges will burn more rapidly insome zones than in others, with the result that the propellant particlesare caused to move axially of the well bore until they encounterpressure build ups sufficient to create new zones of maximum combustion.For example, considering the upper-end ignition of FIG. 5, propellantparticles are driven toward the lower end of this charge due to therelatively high rate of combustion adjacent the upper end thereof.Ultimately, the downwardly moving propellant particles are subjected toa sufficient pressure increase to produce burning thereof at arelatively high rate, whereupon they surge upwardly again. With theignition system of FIGS. 2 to 4, similar back and forth axial movementof the propellant particles occurs, but not to as great a degree due tothe fact that the charge 28 is ignited progressively at its centerthroughout substantially its entire length. In other words, with theignition system of FIGS. 2 to 4-, the axial amplitude of motion of thepropellant particles is less than with the ignition system of FIG. 5,the same being true of the system of FIG. 10. The igniting means of FlG.7 results in minimum axial movement of the propellant particlesdispersed in the liquid 24 due to the fact that the more intensecombustion zones opposite the nodes 96 act as barriers tending toprevent axial movement of propellant particles past a position oppositethe next node in the string.

The net result of the foregoing combustion process is to producepressure pulsations in the liquid 24 which act on the productiveformation 16 to produce progressive, controlled, generally radiallyoutward fissuring thereof, the pressure pulsations acting through thehole 26 in the casing 18 where the well bore 1% is cased. As shown inFIGS. 3 and 4, fragments 56 of the fragmentized container shell 32 tendto plug the smaller perforation 26 with which the casing 15 may beprovided to intensify the action through the hole or holes 26, thelatter being sufiiciently large that it or they will not be obstructedby such fragments 56. During the combustion process, many of thepropellant particles are ignited, extinguished, reignited, and so on, asthey surge back and forth axially of the well bore between high pressurezones of maximum combustion intensity, thereby prolonging the combustionprocess and the period throughout which the pressure pulsations in theliquid 24- are produced. As indicated in FIG. 3 of the drawings, many ofthe propellant particles may be carried into the fissure, indicated bythe numeral 98, formed in the productive formation 16 and may burntherein to supplement the action of the pressure pulsations in extendingthe fissure. The fact that the propellant particles are relatively smallinsures that they will not be entrained by gases rising through theliquid 24 as the result of the combustion process, and will remain inthe region of the productive formation 16 until substantially all of theparticles are consumed.

Carrying out the combustion process with at least the liquid headhcreinbefore discussed insures the development of pressure pulsations ofsufiicient amplitude to fissure the formation 16 in the desired manner.At the same time, since the combustion process is prolonged as theresult of ignition, extinguishment, reignition, reextinguishment, and soforth, of the propellant particles, and since the liquid 24 isunconfined to provide in effect an air or gas-containing surge chamberthereabove, the development of excessive pressures is prevented.Probably, the pressure pulsations do not exceed short-duration peakvalues of 50,000 p.s.i., which are sufficient to produce the desiredprogressive fissuring of the formation 16, but which are insufiicient todamage the casing 18, or shatter or compact the formation 16.

The combustion rate may also be rendered irre ular, and at the same timereduced, through the use of a filler or matrix which partially orcompletely encases the propellant particles. FEG. 8 of the drawingsshows in fragmentary form a charge 10% composed of a matrix 192 andpropellant particles 104 the bulk of which are in contact with otherparticles. In FIG. 9 is shown in fragmentary form a charge 106comprising a matrix 108 having propellant particles 110 embeddedtherein, the majority of the particles 110 being out of contact witheach other. With charges like the charges and 105, initial ignitionbreaks up the matrices 102 and 1% to expose at least some of theparticles and 108 for ignition. The particles res and thus ignitedreduce the matrix pieces formed by initial ignition of the charges intostill smaller pieces, and this process continues until all, orsubstantially all, of the particles 1% and 11% are exposed and ignited.Thus, the matrices 192 and 16% prolong combustion and result inirregular combustion rates. As will be apparent, with the charge 1%,combustion proceeds less irregularly and at a more rapid rate than withthe charge 1% due to the fact that, with the charge 100, combustion mayproceed directly from one particle 104 to the next in most instances.With the charge 1&6, on the other hand, combustion progresses moreslowly and is more irregular since after each particle 11b is exposed,it is not ignited until it encounters another, burning particle.

With a charge of the matrix type, such as the charges 10% and 106, anydesired degree of contact between the propellant particles may beprovided to produce any desired pressure-time curve. If, for example, aplaster of Paris matrix is used, the plaster of Paris in liquid form ispoured into a mold, which may be the final charge container, filled withpropellant particles under varying degrees of pressure. If thepropellant particles are under relatively high pressure when the plasterof Paris, or other matrix material in liquid form, is poured into themold, virtually all of the propellant particles will be in contact aftersetting of the plaster of Paris. On the other hand, by applying littleor no pressure to the mass of propellant particles in the mold as theliquid plaster of Paris is introduced, virtually complete separation ofthe propellant particles by the matrix may be obtained.

An important feature of the combustion process of the invention is thatthe gaseous products of combustion are not forced to concentrate in thevicinity of the productive formation 16 as it is fissured, therelatively slow combustion rate resulting from dispersion of propellantparticles throughout the liquid 2- 5- adjacent the productive formation,either as free particles or as matrix-encased particles, providing ampletime for the gases resulting from combustion of initially-ignitedparticles to rise through the liquor prior to the formation of gaseousproducts of combustion by subsequently ignited particles. Thus, thegases rise through the liquid 2-6 in a steady stream as the combustionprogresses. The net result of this is that the fluid present in thevicinity of the productive formation 16 as it is being fissured by thepressure pulsations in the liquid 24- is largely liquid, and contains arelatively small proportion of gaseous products of cornbustion.Consequently, the fluid surging into the fissure 93 as it isprogressively extended by the pressure pulsations contains a largeproportion of well fluid thereby reducing the tendency to heat theexposed surfaces of the fissure to a temperature sufficiently high tofuse them, which would have a decidedly adverse etfect on permeability.Thus, the combustion process of the invention, in addition to producingpressure pulsations of relatively low amplitude, also maintains thetemperatures adjacent the productive formation 16 relatively low toavoid fusion damage to such formation.

The disclosure thus far has been concerned with details of thecombustion process of the invention and details of the propellantcharges thereof, including descriptions of the manners in which thevarious charges are ignited In subsequent paragraphs, the manner inwhich the charge 28 is lowered into the well bore, supported therein,and the igniting means thereof actuated, will be considered.

Referring to FIGS. 1 and 2, the firing head 34, which is shown assecured to the shell 32 by screws 112 and sealed with respect thereto byan O-ring 114, carries radially outwardly and upwardly extending,cantilever springs 1'16 which are engageable with the casing 13, or thewall of the well bore 1b, to support the charge 28 after it has beenlowered into position opposite the productive formation 16 to befissured. The firing head 34', is provided at its upper end with anupwardly extending skirt 118 into which 1% a coupling 12% extends. Thefiring head 34 is sealed relative to the coupling by an O-ring 122carried by the coupling and engaging the skirt 118, and is secured tothe coupling by a pin 124 located close to the upper end of the skirt113 and extending through the skirt into the coupling.

The upper end of the coupling 120 is threaded into a sinker bar 126having sutficient weight to force the charge 23 downwardly through thecasing :18 in opposition to friction between the springs 116 and thecasing 18 and in opposition to fluid resistance. Connected to the upperend of the sinker bar 126 is a wire line 128 by means of which thesinker bar and the charge 28 may be lowered into the desired position.The wire line 128 contains an electrical conductor 130, shown in FIG.'2, which extends downwardly through the sinker bar 126 and which isconnected at its lower end to a contact 132. The coupling 12d carries acontact 134 which electrically engages the contact 132; when thecoupling and the sinker bar 128 are assembled. The contact 134 isconnected to an ignitor 136 which is fired when current is caused toflow through the conductor by closure of a switch, not shown, at thesurface, the ignitor being grounded to complete the circuit. Firing ofthe ignitor 136 results in ignition of an auxiliary explosive charge 140in a bore 142 through the coupling 12d, combustion of the charge 1 thresulting in the development of pressure within the skirt 118 below thecoupling 12 9.

Such pressure within the skirt 118 acts on the upper end of a piston 144disposed in a counterbore 145 in the firing head 34 which is coaxialwith the bore 66 therein, the piston being sealed by an O-ring 148 andnormally being retained in an upper, inoperative position by a shear pin150, or other retaining means, engaging the bottom wall of the recessdefined by the skirt 118. The pressure developed by combustion of thecharge 148 causes failure of the shear pin 15% with the result that thepiston 144 is propelled downwardly in the counterbore 146. The piston144i is provided at its lower end with a firing pin 15?. which strikesthe igniter 72 as the piston reaches the lower end of the counterbore146, the ignitor 72, as hereinbefore discussed, initiating combustion ofthe delay train 74.

The pressure produced by combustion of the charge see also separates thecoupling 1% from the firing head 34- by rupturing the skirt 11% abovethe pin 124, as shown in FIG. 3. Thereupon, the sinker bar 126 isWithdrawn from the well, or is at least moved upwardly therein asubstantial distance above the charge 28, by means of the wire line 128,the delay train 74 providing suificient time to permit raising thesinker bar 126 at least a distance surficient to avoid its interferingwith the rise of gas in the fluid column and to avoid exxcessiveslackening of the wire line.

Referring to FIG. 5 of the drawings, the embodiment illustrated thereinis similar to that illustrated in FIGS. 2 to 4 and includes a coupling169 connected to a skirt 162 on the firing head 42 by a pin vlo l andconnected to a sinker bar 166. The latter carries a conductor 163 whichis connected to an ignitor 17% through contacts 172 and 174, thecoupling containing an auxiliary explosive charge 176 which is ignitableby the ignitor 176 to separate the sinker bar 166 from the firing head42. A counterbore 178 in the firing head 42 contain a piston 18bcarrying a firing pin 182 for setting off the ignitor S8. The upper endof the piston 130 is exposed to the charge 176 in the same manner thatthe piston 144 is exposed to the charge 144), the piston 13% being heldin an inoperative position by a shear pin 184 which fails as the resultof combustion of the charge 176.

The charge 46 is provided with a supporting means 1&6 engageable withthe wall of the well bore 10, or with a casing, which includes coilsprings 188 having arms 1% anchored to the firing head 42 and arms 192frictionally engageable with the wall of the Well bore. A sleeve 194threaded on the lower end of the sinker bar the normally holds the arms1192 in retracted, inoperative positions, shown in solid lines, thesleeve 194 being threaded downwardly over the springs 1&8 to hold thearms 192 in their retracted positions after assembly of the sinker bar,the coupling 160 and the firing head 4-2. Upon separation of the firinghead 42 and the coupling 166' as the result of combustion of the charge176, the sleeve H4 disengages the springs 133 to permit the arms 192thereof to fly outwardly into engagement with the wall of the well boreto support the charge 46, such positions of the arms 192 being shown inbroken lines. The action of the springs 183 is so rapid that the charge46 drops but a very short distance before the arms 1%2 engage the wallof the well bore to support the charge in the desired position. Duringthe time interval provided by the delay train 1 the sinker bar 166 isdrawn upwardly a substantial distance, or to the surface, beforeignition of the charge 55.

Considering the over-all operation of the invention, with the properhead of liquid 24 in the Well bore 10' above the productive formation 16to be fissured, and after forming the hole 26 in the casing 18 in theevent that the well bore is eased, the charge of propellant explosive islowered into a position opposite the productive formation by means ofthe wire line and the sinker bar. The small auxiliary explosive chargein the coupling connecting the main charge and the sinker bar is thenfired, which results in delayed ignition of the main charge by thecorresponding piston 144 or 122i and results in detaching the maincharge from such coupling. The main charge is then supported in theproper position either by the springs 116, or the supporting means 186,the latter being activated by separation of the charge from itssupporting coupling and sinker bar.

As soon as the charge has been detached from its supporting coupling andsinker bar, and during the period provided by combustion of the delaytrain 74 or hit, the sinker bar is moved upwardly, by means of the wireline attached thereto, either to the surface, or at least to asufficient distance above the charge to prevent damage thereto and toprevent interference by the sinker bar with upward movement of gasesresulting from combustion of the chargev Upon expiration of the desiredtime delay, the delay train ignites the main charge, such ignitiontaking place at the upper end of the charge in the case of the charge46, taking place throughout substantially the entire length of thecharge in the case of the charges 23 and 55, and ignition of the chargetaking place with a maximum intensity at a plurality of axially spacedpoints with the charge 54. Upon ignition, the charge is dispersedthroughout the liquid 24 adjacent the productive formation 16 to befissured, the container in which the charge is disposed beingfragmentized upon ignition of the charge. The propellant particles aredispersed laterally throughout the liquid 24- as a result of bursting ofthe charge container and then move axially in the well bore Ell awayfrom zones of maximum combustion intensity due to the higher pressuresexisting in such zones, until the axial movement of the propellantparticles results in pressure build ups in new zones sufficient tocreate new points of maximum combustion intensity. This axial migrationof the propellant particles takes place in both directions to produceconstantly shifting zones of maximum combustion intensity, the axialmovement of the propellant particles being a maximum with ignition ofthe charge at one end only, and being a minimum with the type ofignition provided by the charge 54 of PEG. 7 since the more intenseignition zones provided by the nodes as limit axial movement of thepropellant particles. During the process of migrating back and forth,the propellant particles may be ignited, at least partiallyextinguished, reignited, and reextinguished many times beforesubstantially complete combustion occurs, and some of the propellantparticles may actually be swept l2 into and out of the fissure 93 formedby the combustion process and some combustion may take place therein.

The net result of the foregoing combustion process is the creation ofpressure pulsations in the liquid 24 adjacent the productive formation16, and in the liquidemulsion formed by the explosion, sufficient toform and progressively extend the fissure 98 as these pulsations act onthe formation, but insufficient to shatter or otherwise destructivelydamage the formation. The fissure )8 is frequently extended to pointsmany feet from the well bore to produce substantial increases in thepermeability of the productive formation 16, and thus substantialincreases in the production of fluid therefrom.

Due to the fact that the combustion of the propellant particles takesplace while these particles are dispersed in the liquid 24 for the mostpart, the entire combustion process takes place without the developmentof excessive temperatures which would tend to detract from thepermeability of the productive formation 16, as by fusing the surfacesof the fissures 98. The products of combustion resulting from burning ofthe individual propcllant particles constantly rise upwardly through theliquid 24 and, in so doing, create a pressure reservoir which, duringthe final phases of the process, produces a prolonged downward flow offluid into the fissure or fissures to further extend them. Entrainmentof the propellant particles in the rising gases is minimized due to therelatively small particle sizes.

Thus, the present invention provides a method of and apparatus foreffectively increasing the production of tight productive formations byfissuring such formations in a controlled manner while at the same timeavoiding damage to the well bore, or any casing therein, by avoidingexcessive combustion pressures and temperatures. Although exemplaryembodiments of the invention have been disclosed herein for purposes ofillustration, it will be understood that various changes, modificationsand substitutions may be incorporated in such embodiments Withoutdeparting from the spirit of the invention as defined by the claimswhich follow.

I claim:

1. A method of fissuring a productive formation into which a well boreextends, and which is submerged in liquid in the well bore, to increasethe permeability of the formation and thus the flow of fluid from theformation into the well bore, including igniting and dispersing in theliquid in the well bore adjacent the productive formation in discreteform an explosive of the propellant type, whereby the explosive issubjected to irregular combustion so as to produce pressure pulsationsin the liquid adjacent the productive formation as it burns to fissurethe productive formation in a controlled manner.

2. A method of fissuring a productive formation into which a well boreextends, and which is submerged in liquid in the well bore, to increasethe permeability of the formation and thus the flow of fluid from theformation into the well bore, including dispersing in the liquid in thewell bore adjacent the productive formation particles of an explosive ofthe propellant type at least some of which are burning, whereby theexplosive is subjected to irregular combustion so as to produce pressure pulsations in the liquid adjacent the productive formation as itburns to fissure the productive formation in a controlled manner.

3. A method of fissuring a productive formation into which a well boreextends, and which is submerged in liquid in the well bore, to increasethe permeability of the formation and thus the flow of fluid from theformation into the well bore, including the steps of: submerging in theliquid in the well bore adjacent the productive formation an explosiveof the propellant type; and producing irregular combustion of saidexplosive so that it produces pressure pulsations in the liquid adjacentthe productive formation as it burns to fissure the productive formationin a controlled manner.

4. A method of fissuring a productive formation into which a well boreextends, and which is submerged in liquid in the well bore, to increasethe permeability of the formation and thus the flow of fluid from theformation into the well bore, including the steps of: submerging in theliquid in the well bore adjacent the productive formation an explosiveof the propellant type; and igniting said explosive in an irregularmanner so that the explosive produces pressure pulsations in the liquidin the well bore adjacent the productive formation as it burns tofissure the productive formation in a controlled manner.

5. A method of fissuring a productive formation into which a well boreextends, and which is submerged in liquid in the well bore, to increasethe permeability of the formation and thus the flow of fluid from theformation into the Well bore, including the steps of: submerging in theliquid in the well bore adjacent the productive formation an explosiveof the propellant type; igniting the explosive nonuniforrnly; anddispersing the explosive in the liquid in the well bore adjacent theproductive formation so that the explosive produces pressure pulsationsin the liquid adjacent the productive formation as it burns to fissurethe productive formation in a controlled manner.

6. A method of fissuring a productive formation into which a well boreextends to increase the permeability of the formation and thus the flowof fluid from the formation into the well bore, including the steps of:providing in the well bore above the productive formation a head ofliquid at least equivalent to 200 feet of a liquid having a specificgravity in the neighborhood of unity; submerging in the liquid in thewell bore adjacent the productive formation a readily dispersible chargeof an explosive of the propellant type having a diameter considerablyless than the diameter of the well bore and having lengthdiameter ratioof at least 3:1; and igniting said charge non-uniformly so as todisperse it in the liquid in the well bore adjacent the productiveformation, whereby the explosive scattered throughout the liquidadjacent the productive formation produces pressure pulsations thereinas it burns to fissure the productive formation in a controlled manner.

7. A method of fissuring a productive formation into which a well boreextends to increase the permeability of the formation and thus the flowof fluid from the formation into the well bore, the well bore havingtherein a casing which extends into the productive formation, saidmethod including the steps of: forming in the casing opposite theproductive formation at least one hole having an effective diameter ofat least one inch; providing in the casing above said hole a head ofliquid at least equivalent to 200 feet of liquid having a specificgravity in the vicinity of unity; submerging in the liquid in the casingopposite said hole a readily dispersible charge of an explosive of thepropellant type having a diameter considerably less than the insidediameter of the casing and having a lengthdiarneter ratio of at least 3:1; and igniting said charge nonuniformly so that combustion of portionsof the explosive disperse the charge in the liquid in the casingadjacent said hole, whereby the scattered explosive produces pressurepulsations in the liquid in the casing adjacent said hole as it burnswhich act through said hole to fissure the productive formation in acontrolled manner.

8. A method of fissuring a productive formation into which a well boreextends, and which is submerged in liquid in the well bore, to increasethe permeability of the formation and thus, the flow of fluid from theformation into the well bore, including progressively burning successiveportions of a propellant explosive which is dispersed in particle formin the liquid in the well bore adjacent the productive formation so asto produce a series of pressure pulsations in the liquid adjacent theproductive formation as successive portions of the explosive areprogressively burned to fissure the productive formation in a controlledmanner.

References Cited in the file of this patent UNITED STATES PATENTS855,224 Broadwater May 28, 1907 2,031,505 Rison Feb. 18, 1936 2,080,875Pitzer May 18, 1937 2,214,227 English Sept. 10, 1940 2,619,180 Smith etal Nov. 25, 1952 2,676,662 Ritzmann Apr. 27, 1954 2,696,258 Greene Dec.7, 1954 2,696,259 Greene Dec. 7, 1954 2,703,528 Lee et a1 Mar. 8, 19552,712,355 Hoff July 5, 1955 2,756,826 Ebaugh July 31, 1956 2,766,828Rachford Oct. 16, 1956 2,790,388 M'acLeod Apr. 30, 1957 i a V I r

8. A METHOD OF FISSURING A PRODUCTIVE FORMATION INTO WHICH A WELL BOREEXTENDS, AND WHICH IS SUBMERGED IN LIQUID IN THE WELL BORE, TO INCREASETHE PERMEABILITY OF THE FORMATION AND THUS THE FLOW OF FLUID FROM THEFORMATION INTO THE WELL BORE, INCLUDING PROGRESSIVELY BURNING SUCCESSIVEPORTIONS OF A PROPELLANT EXPLOSIVE WHICH IS DISPERSED IN PARTICLE FORMIN THE LIQUID IN THE WELL BORE ADJACENT THE PRODUCTIVE FORMATION SO ASTO PRODUCE A SERIES